jbd: don't wake kjournald unnecessarily

Don't send an extra wakeup to kjournald in the case where we
already have the proper target in j_commit_request, i.e. that
commit has already been requested for commit.

commit d9b01934 "jbd: fix fsync() tid wraparound bug" changed
the logic leading to a wakeup, but it caused some extra wakeups
which were found to lead to a measurable performance regression.
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/jbd/journal.c

@@ -446,7 +446,8 @@ int __log_start_commit(journal_t *journal, tid_t target)
 	 * currently running transaction (if it exists).  Otherwise,
 	 * the target tid must be an old one.
 	 */
-	if (journal->j_running_transaction &&
+	if (journal->j_commit_request != target &&
+	    journal->j_running_transaction &&
 	    journal->j_running_transaction->t_tid == target) {
 		/*
 		 * We want a new commit: OK, mark the request and wakeup the